In the past a number of devices and methods have been used addressing the problem of mine roof control. Perhaps the best summary of the current state of the art is in the Bureau of Mines Information Circular (1986), designated IC 9094 and entitled Cutter Roof Failure: an Overview of the Causes and Methods for Control, the same having a principal author of John L. Hill III and published by the United States Department of the Interior. The disclosure therein is fully incorporated by way of reference. Also pertinent are the inventor's prior patent applications entitled Mine Truss Structure and Method, Ser. No. 06/809,140 filed Dec. 16, 1985 and allowed Patent Application entitled Truss Systems and Components Thereof, Ser. No. 06/809,139, filed Dec. 16, 1985, both of these patent applications also being incorporated herein by way of reference. No hardware or method are believed known, nor does the inventor believe himself aware of any patent or other literature, wherein angle-bolting hardware as provided herein includes means lateral tension securement of mutually-spaced, end-to-end facing adjacent devices such that roof strata in a mine be placed in compression by incorporation of angle bolting devices and tensioned tie rods connecting the same, proximate the area where cutter failure is likely to occur. Nor is there known any truss structures incorporating such bracket devices wherein the elongate, rib adjacent or proximate areas of the roof can be pre-stressed in compression to overcome the tension of force fields naturally occurring in the strata proximate the entry corner areas. In said Bureau of Mines Information Circular, anchor-bolting hardware and anchor bolt types are illustrated in FIGS. 25 and 27 of the publication. However, in cases of conditions leading to severe cutter roof failure, the installations alone of angle bolts and their brackets are not sufficient to overcome the dangerous condition present and to prospective cutter failure. The cribbing and posts are generally used currently in the art for the support of the roof after cutter failure has formed; cribs and posts, of course, should be installed shortly after mining, and the support should be positioned in the mine in such a way as not to yield significantly.
Another problem that inheres in present practice is the fact that cutter failure frequently propogates at areas just above the anchor horizon of the longest anchor bolt; and this can result in massive roof failure. Even the installations of successive rebar-type bolts with the point-anchoring feature, and even though such are uniformly tensioned, do not deter the severe problem chanced through the appearance of cutter fractures.
For convenience, and by way of definition, the following may be taken as an approximate definition of cutter roof failure:
Cutter Roof Failure, according to the Bureau of Mines Publication, in mine roof rock "is a failure process that initially begins as a fracture point in the roof rock parallel to, and located at, the roof-rib intersection."
Such fractures occur through a wide variety of angles, generally will be from a vertical position proximately in line with the rib line to approximately 30 degrees relative to the horizontal and oriented over the roof. A customary fracture approximate 60 degrees relative to the horizontal defined by the roof, extending over the opening. Once initiated along the roof-rib line or its extrapolation, a cutter fracture may propagate away from the rib-roof line and actually cross the roof's span, continuing the other side of the room. Various modes of failure are illustrated in the Bureau of Mines Publication.